Ca isotope variations in Allende

We present Ca isotopic measurements on Allende Ca-Al-rich inclusions (CAIs) and an apatite enriched fraction from Orgueil. The results on CAIs show widespread excesses on the neutron-rich isotope ⁴⁸Ca. All other isotopes agree with the normal within our presently obtainable precision. Seven out of 11 CAIs analyzed exhibit isotopic anomalies ranging up to ~ +6ɛ units (1ɛ = 1 part in 10,000). This abundant occurrence of isotopic excesses places Ca alongside Ti and O, elements which show isotopic anomalies in all Allende CAIs measured so far. However, at present no clear correlation can be found between excesses in ⁴⁸Ca and ⁵⁰Ti, the isotopes which are thought to be coproduced by neutron-rich nucleosynthetic processes within stars. We believe that the relatively higher volatility of Ca compared to Ti compounds could have led to a variable dilution with isotopically "normal" Ca in vaporization and recondensation processes in stellar envelopes, the interstellar medium and/or the solar nebula. High precision measurements of ⁴⁶Ca limit possible anomalies in this 33 ppm abundance isotope to 10ɛ units or less and, together with the observed ⁴⁸Ca and ⁵⁰Ti anomalies, constrain possible nucleosynthetic mechanisms capable of producing these neutron-rich nuclei.